In long distance learning, presentation material is displayed on a screen or board, and the presenter stands to the side or at times in front of the board or screen, talking about the subject and pointing to illustrations or text on the screen that would enhance the effectiveness of his presentation. In order to encode and transmit a visual of his lecture to a distant location, the presentation material is sent electronically, preferably from its original digital source if available, rather than a likely degraded “second generation” image from a camera capturing the original presentation screen.
However, it is also desired that the presenter be shown as well, so as to make the material more relatable on a human level, through his gestures, social cues, and real time reaction to the material, and to the audience, bringing the lecture to life. At the remote distant location, the presenter is shown along with the presentation material. This is typically done by displaying the presentation material on part of, or all of, the receiver's screen, with the presenter in a separate frame or “window”, typically displayed on a corner of the screen. The presenter can also be composited over the presentation material to appear to be standing in front of it. The compositing of a foreground over a background has many methods of implementation. Foreground elements are isolated from an image so that what surrounded that element is replaced by the background image. Foreground elements will typically have both opaque and transparent areas that need to be identified and preserved in the finished composite.
Numerous techniques have been developed to facilitate compositing. Blue screen and green screen, front projection retro-reflective photography, sodium light, infra-red, ultra violet, chroma-key, stereo pair images, depth keys, difference keys and many other matte extraction techniques are well known and employed every day.